1. Field of the Invention
The present invention relates to an oscillator for generating a high-frequency signal and to a high-frequency superposing module including such an oscillator for driving a laser diode.
2. Description of the Related Art
A laser-diode drive circuit is one of various applications of a high-frequency oscillating circuit. In a conventional laser-diode drive circuit, a laser diode is driven in a multiple mode where a high-frequency current is superposed on a driving DC current, thereby preventing the generation of mode-hopping noise resulting from an increase in the temperature of the laser diode. For superposing a high-frequency current on a DC current, an oscillating circuit that generates a high-frequency signal is used as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-93758.
FIG. 8 is a circuit diagram of a laser-diode drive circuit disclosed in Japanese Unexamined Patent Application Publication No. 7-93758. In this circuit, a DC current is supplied via an input terminal LDA to a laser diode LD and power is supplied to an oscillating circuit 2 via a power input terminal VCC. Resistors R1, R2, and R3 apply a predetermined DC bias to a transistor Q1. This oscillating circuit 2 is a Colpitts oscillator and its oscillation frequency is determined by the values of capacitors C3, C4, and C5 and inductors L1 and L3. The oscillation output of the oscillating circuit 2 is passed to the laser diode LD via a matching circuit composed of capacitors C6 and C7. FIG. 8 also shows an oscillation control terminal RMS and a ground terminal GND. When the oscillation control terminal RMS has a potential equal to that of the power input terminal VCC, a predetermined bias voltage is applied to the base of the transistor Q1 to start oscillation. When the oscillation control terminal RMS is open, the potential at the base of the transistor Q1 is substantially 0, thus stopping oscillation.
A laser-diode drive circuit as described above requires the oscillating circuit to be turned ON/OFF. For example, the oscillating circuit would be turned ON to superpose a high-frequency signal for the laser diode when data is read from a DVD or turned OFF to stop superposing a high-frequency signal when data is written to the DVD. Unfortunately, the conventional laser-diode drive circuit shown in FIG. 8 takes a long time until the oscillating circuit 2 starts oscillation. This mechanism is described below. As shown in FIG. 8, the oscillating circuit 2 has the base line of the transistor Q1 connected to the ground level via the resistor R2 when the oscillation control terminal RMS is open (that is, no voltage is applied to the oscillation control terminal RMS). In this structure, when the oscillation control terminal RMS is switched to have a potential equal to that of the power input terminal VCC (power supply voltage) to start oscillation, the oscillating circuit 2 needs to increase the potential at the base line of the transistor Q1 from the ground level to the required level. In other words, it takes some time until the capacitors C3, C4, and C5 are charged to increase the potential at the base of the transistor Q1 to the required level.
For a system that requires oscillation to be activated/deactivated within a short period of time for a laser diode, slow response of the oscillating circuit adversely affects the response of the entire system. In short, slow activation of the oscillating circuit leads to an undesirable state where reading from and writing to a DVD require a long time.
The oscillating circuit 2 has been described as applied to a laser-diode drive circuit. However, the same problem occurs in applications that require a quick response in the activation/deactivation of oscillation.